The field of the present invention relates to a magnetic resonance imaging apparatus, a scanning device, a magnetic resonance imaging method and a program therefor, and more particularly to a magnetic resonance imaging apparatus which so executes a scan to collect magnetic resonance signals from a subject in an imaging space wherein a magnetostatic field is formed as to match the PROPELLER (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) method and generates an image of the subject on the basis of the magnetic resonance signals obtained by the execution of the scan, a scanning device and a magnetic resonance imaging method by which that scan is executed, and a program therefor.
A magnetic resonance imaging (MRI: Magnetic Resonance Imaging) apparatus is known as an apparatus that picks up images of a subject by utilizing the nuclear magnetic resonance (NMR: Nuclear Magnetic Resonance) phenomenon, and is used in medical, industrial and many other fields.
When a subject is to be imaged by using a magnetic resonance imaging apparatus, first the subject is accommodated in the imaging space wherein a magnetostatic field is formed, and the spinning directions of protons in the subject are aligned with the direction of the magnetostatic field to achieve a state in which a magnetization vector is obtained. After that, the magnetization vector of the protons in the subject is flipped by irradiating the subject with an RF pulse, which is an electromagnetic wave of the resonance frequency, from an RF coil and thereby causing a phenomenon of nuclear magnetic resonance to arise. The magnetic resonance imaging apparatus then returns to the original magnetization vector. Magnetic resonance signals deriving from the protons in the subject are received by the RF coil. On the basis of the received magnetic resonance signals, for instance, a sliced image is generated regarding a sliced plane of the subject.
If a bodily movement of the subject occurs here when executing the scan to collect magnetic resonance signals from the subject, a bodily movement artifact may arise in the image generated as described above. For instance, a bodily movement artifact may occur in the phase encode direction and invite a trouble of deteriorated image quality. This trouble is mainly attributable to sampling of magnetic resonance signals at every time of repetition (TR) in the frequency encode direction of the k space to successively match the matrix in the phase encode direction of the k space.
The PROPELLER method is known as a method of restraining the occurrence of such bodily movement artifacts.
The PROPELLER method is a method of so sampling magnetic resonance signals required for image reconstruction as to match the k space by turning a strip-shaped blade in the central part of the k space. Thus, it is a sampling method to which the concept of phase encode is applied among methods of radially sampling magnetic resonance signals matching the k space. More specifically, by performing the action to so collect repetitively the magnetic resonance signals as to match a blade containing a plurality of trajectories in each of the positions to which the blade is successively turned around the center of the k space at intervals, the magnetic resonance signals are so sampled as to match each matrix of the k space.
As this method is to scan regions of a k space whose central part is the same as the k space at the time of collecting divided sets of data, the data can be used as two-dimensional navigator data, and accordingly accurate compensations for bodily movements can be achieved. Since magnetic resonance signals matching the low frequency region in the k space are successively collected in duplication, the bodily movements of the subject can be detected on the basis of each of the magnetic resonance signals collected in duplication. As a result, as the bodily movements can be compensated for by using this result, the image quality can be enhanced (see Patent Document 1 and Non-Patent Document 1 for instance).    Patent Document 1. Japanese Unexamined Patent Publication No. 2004-237109.    Non-Patent Document 1. Magnetic Resonance in Medicine 42:963-969, 1999, James G. Pipe, Motion Correction with PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging.
In sampling by this PROPELLER method, as high speed data collection is required to prevent any blade, usually magnetic resonance signals are so collected repetitively as to match the trajectories of each blade by the FSE (fast spin echo) method. For instance at 160 to 200 ms intervals, magnetic resonance signals are collected. This FSE method, as it gives a T2 effect by increasing the number of echo trains, enables an appropriate T2 emphasized image to be generated.
However, in order to prevent a T2 effect to arise when a T1 emphasized image is to be generated in this process, it is difficult to increase the number of echo trains. Accordingly, effective collection of many magnetic resonance signals is made difficult, and high speed data collection may become difficult to achieve.
Along with that, it is also made difficult to collect many data for compensation for bodily movements, possibly resulting in difficulty of effectively correcting bodily movement artifacts and to generate a T1 emphasized image of high image quality.
Since high speed data collection cannot be easily accomplished in sampling by the PROPELLER method when the FSE method is applied, it is difficult to appropriately compensate for bodily movements and to generate a T1 emphasized image of high image quality.